In addition to endoscopes for medical and non-medical technical applications, whose viewing angle is parallel to the longitudinal axis of the endoscope shaft, endoscopes with other fixed viewing angles have been developed for some time. The viewing angle of an endoscope is understood here and hereinafter always to mean the direction facing from the distal end of the endoscope, in which an object is situated that appears in the center of the image recorded by means of the endoscope. In many applications, however, a fixed viewing angle is a disadvantage. In the worst case, for example during a medical procedure, the endoscope must be replaced numerous times. In such cases it is an advantage to use an endoscope with a viewing angle that can be selected or adjusted in situ.
Observing an object in a cavity by means of an endoscope assumes as a rule that there is some illumination of the object. For this purpose an endoscope comprises, for example, lightwave conductors, in particular glass fibers, by means of which an illuminating light is transmitted from the proximal end of the endoscope along the shaft to the distal end of the endoscope. Light outlet surfaces of the lightwave conductors on the distal end of the endoscope are positioned and configured in such a way that the entire visual field or viewing field is sufficiently and homogeneously illuminated.
In an endoscope with adjustable viewing angle, the illuminating light on the distal end of the endoscope, in the simplest case, is distributed in such a way that the entire visual field is illuminated independently of the particular viewing angle selected. This results, however, in a series of disadvantages. In particular, light capacity is wasted, because the entire visual fields of all selectable viewing angles are illuminated constantly, independently of the viewing angle that is actually selected. Thus, at a predetermined desired brightness, a markedly higher lighting capacity must be provided altogether than with an endoscope with a fixed viewing angle.
An additional disadvantage arises from the fact that illuminating light of high intensity can photo-thermally or photo-chemically damage tissue or other objects. With an endoscope with fixed viewing angle, the distal end of the endoscope is at too close a distance to an object, at least on observing the recorded image. In using a video camera on the endoscope, an automatic warning of users is also possible if the brightness of a recorded image exceeds a predetermined threshold. With an endoscope with adjustable viewing angle, however, part of the illuminating light impinges on objects lying outside the visual field. Therefore there is no undesired approach of the distal end of the endoscope to these objects, and no resulting illumination of these objects with too high a radiant capacity.
A further disadvantage consists in the fact that illuminating light radiated outside the visual field can also be dispersed or reflected by objects or opaque media. The reflected or dispersed illuminating light can arrive directly or indirectly in the observation beam path. Consequently, contrasts and the distinguishability of objects, especially in dark image areas, can be reduced.
An additional disadvantage comes from the fact that the illuminating intensity or intensity of the illuminating light is essentially constant in the direction in which the viewing angle can be varied (often referred to also as the vertical direction), while it decreases slightly as a rule toward the edge of the visual field in the direction perpendicular thereto (often also called the horizontal direction). However, users of endoscopes with fixed viewing angle are as a rule accustomed to an illuminating intensity that slightly declines toward the edge of the visual field both in the horizontal and in the vertical directions. The illuminating intensity that is constant in the vertical direction can therefore be experienced as an irritant.
Patent application DE 600 15 375 T2 describes an arrangement of several prisms. One of the prisms can be rotated around an axis in order to cast illuminating light at an adjustable viewing angle. The prisms, however, require a structural space that is not available in every endoscope. In addition, it can become burdensome in practice, with the arrangements described in DE 600 15 375 T2, to achieve simultaneously a strong degree of optical isolation of the illuminating and observation beam paths, considerable light strength in the observation beam path, low losses in the illuminating beam path and a small shaft diameter.